Scientists at the European Southern Observatory in Chile have announced the discovery of a new planetary system, composed of a trio of Neptune-like planets 41 light-years away. The discovery, they say, represents a 'Rosetta stone' in understanding of how planets form. Over a period of two years, the astronomers at the Observatory studied HD 69830, a star less massive than the Sun but just visible to eye in the Puppis constellation. Using the High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph on the ESO telescope, the astronomers detected the presence of three 'tiny companions' orbiting their parent star. While the inner two planets appear to be composed mainly of rock, the astronomers say that the third is likely to be made of a rocky/icy core surrounded by an envelope of gas. All three planets are of equal mass, comparable with that of the planet Neptune. Although too big to be considered Earth-like, being ten to 18 times heavier in mass, the outer planet is located near the inner edge of a habitable zone, where liquid water can exist at the surface. 'This alone makes the system already exceptional,' says Willy Benz, co-author of a paper published in Nature. The astronomers also observed the presence of an asteroid belt, another property which the newly-discovered system shares with our own solar system. 'The planetary system around HD 69830 clearly represents a Rosetta stone in our understanding of how planets form,' said Michel Mayor, HARPS' Principal Investigator. 'No doubt it will help us better understand the huge diversity we have observed since the first extra-solar planet was found 11 years ago.'